This invention relates to wavelength dispersing and combining gratings for spectroscopy and data transmission applications. More particularly, the invention relates to gratings that may be used to multiplex and de-multiplex spectrally separated signals into from a single optic fiber.
There exist a number of technical fields (spectroscopy and telecommunications, for example) where it is necessary to spatially separate two or more optical signal which nave different wavelengths or different spectral ranges (or xe2x80x9cwave length rangesxe2x80x9d). In addition, it is often necessary to perform the reverse operation of combining two or more optical signal having different wavelengths or spectral ranges into a single composite signal which is transmitted by a single optic fiber. The first operation is know as de-multiplexing while the second is known as multiplexing. The entire process is known as Wavelength Division Multiplexing (WDM).
A first example of a situation in which devices are required comes from the field of telecommunications. The operating spectral range of a typical optic fiber is between 1100 nm to 1700 nm, which corresponds to a frequency range of 176 THz to 273 THz. Other fibers have a spectral range which begins as low as 800 nm or even lower. Table 1 is a list of some standardized spectral bands for optic data communication.
Although some of these bands have a very large bandwidth, it is not possible to generate a single optical signal which can make use of the entire bandwidth of any band. A typical optical signal comprises a data signal which is modulated onto a carrier frequency, which is the center frequency of the optical signal. The highest modulation frequency which has been achieved thus far is in the range of 1 Thz. Practically achievable modulation frequencies are on the order of 100 Ghz. Therefore, if only modulator is applied to produce an optic signal in any of the spectral bands, only a small portion of the available bandwidth would be used. For example, if a signal with 100 GHz bandwidth is transmitted in the band between 1260 nm and 1360 nm, then only about 0.58% of the available band width will be used.
One known solution to this problem is to multiplex multiple signals with different center frequencies into a single optic fiber which has an operating spectral range broader than the spectral range of the band. The spectral range of the band is divided into a number of non-overlapping channels, ach of which is broad enough to allow a single signal to be modulated at desired frequency without extending beyond the spectral range of the channel. A separate optic signal is produced for each channel and the signals are multiplexed into a transmitting end of the optic fiber.
The combined optic signals are thus transmitted on the single optic fiber and at its opposite receiving end, they are de-multiplexedxe2x80x94i.e. they are separated into separate signal which correspond to the original signal produced for each channel.
In addition to multiplexing a set of separate signals which are spectrally spaced in separate channels with a single spectral band to form a composite signal including of the entire set of signals within the spectral band, it is possible to multiplex to or more such composite signals which have sets of signals from different spectral bands. Thus it is possible to form first composite signal having a plurality of separate signals from the spectral band, for example, between 1260 nm and 1360 nm and a second composite signal having a plurality of separate signals from the spectral band, for example, between 1528 nm and 1561 nm. These two composite signals are formed using two independent multiplexing operations. The two composite signals are then multiplexed onto a single fiber, which may be called a xe2x80x9ctrunkxe2x80x9d fiber, in a third multiplexing operation. At the opposite end of the trunk fiber, the two composite signals are first de-multiplexed from one another and then the separate signals forming each composite signal are de-multiplexed in two independent operations to obtain the original optic signals.
To date, the multiplexing and de-multiplexing operations have generally been performed using reflective relief diffraction gratings, which typically consist of a series of lines etched into the surface of a reflective element. Such diffraction gratings may be either high dispersion or low dispersion devices. A high dispersion reflective relief diffraction grating provides sufficient angular separate between signals in adjacent channels in a single spectral band to separate signals in those channels. A low dispersion reflective relief diffraction grating has a narrower angular separation which is suitable for separating spectrally widely spaced composite signals from different spectral bands.
A high dispersion reflective relief diffraction grating cannot generally be used to separate spectrally widely spaced composite signals (or individual signals). Typically, one or both signals will be poorly diffracted or will not be diffracted at all. For example if a high dispersion reflective relief diffraction grating is used as a de-multiplexer and if the incident signal (which contains two composite signals) is positioned such that one of the composite signals is properly diffracted to allow its separate signals to be separated, then the other composite signal will generally be poorly diffracted at an relatively large angle from the first composite signal.
If a low dispersion reflective relief diffraction grating is used to de-multiplex an incident signal contain two composite signal of different spectral bands, then the individual signals in the separate channels of each composite signal will typically not be spaced sufficiently apart to allow them to received by separate recipient optic fibers or detectors
Other types of gratings, including fibre Bragg diffraction gratings, free-space gratings and echelle gratings, which are used in spectroscopy applications, suffer from various deficiencies.
Accordingly, there is a need for an improved diffraction grating for multiplexing and demultiplexing signals having different wavelengths in an optical communication system.
In one aspect, the present invention provides a n optical coupling device for use with first and second composite signals, said first composite signal having a first spectral range and said second composite signal having a second spectral range, said device comprising a volume diffraction grating having: a substrate; an optically active layer mounted to said substrate; and a structure formed in said optically active layer, wherein said structure is operable in said first and second spectral ranges.
In a second aspect, the present invention provides a volume diffraction grating for multiplexing and demultiplexing first and second composite optical signals, said first composite optical signal having a first spectral range and said second composite optical signal having a second spectral range, said grating comprising: a substrate a first optically active element mounted to said substrate; and a first structure formed in said optically active element, wherein said first structure is operable in at least one of said first and second spectral ranges.
In a third aspect, the present invention provides a volume diffraction grating for multiplexing and demultiplexing first and second composite optical signals, said first composite optical signal having a first spectral range and said second composite optical signal having a second spectral range, said grating comprising; a substrate; an optically active element mounted to said substrate; a first structure formed in said optically active element, wherein said first structure is operable in said first spectral range; and a second structure formed in said optically active element, wherein said second structure is operable in said second spectral range.
In another aspect the present invention provides a mean to improve performance of free space devices for combining (multiplexing) spatially separated spectral components into a single light beam and devices for spatial separation (demultiplexing) of multiplicity of spectral components delivered in a form of a single light beam into separate light beams each of them containing single spectral component by applying suitably optimized volume diffraction gratings.